Air Conditioning Refrigeration Cycle; Heating Cycle - Carrier 68RM35-610 Operation And Service

1.12 AIR CONDITIONING REFRIGERATION
CYCLE
When air conditioning (cooling) is selected by the
controller, the unit operates as a vapor compression
system using R--134a as the refrigerant (see
Figure 1-9).
The compressor raises the pressure and the
temperature of the refrigerant and forces it thru the
discharge lines into the condenser tubes. The
condenser fan circulates surrounding air (which is at a
temperature lower than the refrigerant) over the outside
of the condenser tubes. Heat transfer is established
from the refrigerant (inside the tubes) to the condenser
air (flowing over the tubes). The condenser tubes have
fins designed to improve the transfer of heat from the
refrigerant gas to the air; this removal of heat causes the
refrigerant to liquefy, thus liquid refrigerant leaves the
condenser and flows to the receiver.
The receiver serves as a liquid refrigerant reservoir so
that a constant supply of liquid is available to the
evaporators as needed, and acts as a storage space
when pumping down the system. The receiver is
equipped with sight glasses to observe the refrigerant
level and moisture content.
The refrigerant leaves the receiver and passes through
the liquid/suction heat exchanger. The liquid/suction
heat exchanger further cools the liquid by transferring
heat to the suction gas. The liquid/suction heat
exchanger improves efficiency by delivering the liquid to
the expansion valve in a subcooled state. The
refrigerant then flows to the filter drier which contains an
absorbent that keeps the refrigerant clean and dry.
From the filter--drier, the liquid refrigerant then flows
through the liquid line solenoid valve to the thermostatic
expansion valve. The liquid line solenoid valve closes
during shutdown to prevent refrigerant flow.
T--356
thermostatic expansion valve reduces pressure and
temperature of the liquid and meters the flow of liquid
refrigerant to the evaporator to obtain maximum use of
the evaporator heat transfer surface.
The low pressure, low temperature liquid that flows into
the evaporator tubes is colder than the air that is
circulated over the evaporator tubes by the evaporator
fans. Heat transfer is established from the evaporator
air (flowing over the tubes) to the refrigerant (flowing
inside the tubes). The evaporator tubes have fins to
increase heat transfer from the air to the refrigerant;
therefore the cooler air is circulated to the interior of the
bus.
The transfer of heat from the air to the low temperature
liquid refrigerant in the evaporator causes the liquid to
vaporize. This low temperature, low pressure vapor
passes through the heat exchanger, where additional
heat is transferred to ensure the refrigerant entering the
compressor is in a vapor state. The refrigerant then
continues through the suction line and returns to the
compressor where the cycle repeats.

1.13 HEATING CYCLE

A heating coil is furnished in the unit assembly.
Components furnished by the bus manufacturer may
include a water temperature switch(WTS) and boost
water pump. A solenoid operated reheat coolant valve
may be supplied by the bus manufacturer or by Carrier
Transicold, depending on the application. The controller
automatically controls the reheat coolant valve and
boost pump during the heating and reheat modes to
maintain required temperatures inside the bus. Engine
coolant (glycol solution) is circulated through the
heating circuit by the engine water pump. When the
reheat coolant valve solenoid is energized, the valve will
open to allow engine coolant to flow through the heater
coil. The valve is normally closed so that if a failure
The occurs, the system will be able to cool.
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